Recently, attempts to mount an object detector such as an ultrasonic sensor, a laser radar, an infrared radar, a picture sensor, an electric wave radar, or the like on a vehicle have been made in order to prevent accidents in expressways. However, the ultrasonic radar is influenced by noise, the laser radar or infrared radar is influenced by weather (rain, fog, or snow) or by dust, and the picture sensor is influenced by weather and dust and further has a disadvantage of complicated processing technology or the like, so that they lack in practical use. Contrary to the above, the electric wave radar does not tend to be influenced by weather, and therefore is frequently used for vessels or airplanes. When the electric wave radar is mounted on a vehicle, however, a satisfactory electric wave radar actually can not be obtained, since the electric wave radar accepts clutters from a road surface or surrounding objects.
However, a millimeter wave radar has advantages as described hereinafter. The millimeter wave radar does not tend to be influenced by weather or dust, and is shorter in wavelength as compared with a microwave radar or the like, thereby reducing a transmitting/receiving antenna in size, and consequently the millimeter wave radar can be easily mounted on a vehicle. Specifically, as shown in FIG. 21, the antenna beam width .theta. can be narrowed, which reduces receipt of clutter from a road surface and surrounding objects. Moreover, relative velocity relative to objects can be detected with high precision by Doppler effect. Thus, there is an attempt wherein the millimeter radar is mounted on a high speed traveling vehicle. As shown in FIG. 21, a transmitted wave 3a is transmitted from a transmitting antenna 2a of a transmitting/receiving antenna 2 of the millimeter wave radar mounted on a vehicle 1, and a reflected wave 3b from an object 4 is received by a receiving antenna 2b. Thus, a distance L from the vehicle 1 to the object 4 and a relative velocity V between the vehicle 1 and the object 4 are detected.
As described above, the millimeter wave radar has advantages of being able to be reduced in size and not tending to be influenced by weather, thereby detecting objects with high precision, which can not be obtained by other detectors (the aforesaid ultrasonic sensor and the like). However, only an attempt to detect the distance L and the relative velocity V has been made, but no further attempt is made.
Incidentally, research on fleet running relative to a plurality of unmanned vehicles have been enthusiastically conducted in mines, quarries, and the like. Also in this case, the millimeter wave radar is considered to be effective in being mounted on these vehicles in view of the aforesaid advantages. However, a vehicle with a millimeter wave radar for operating in mines, quarries, and the like is strongly requested to detect meaning of information which an object to be detected has, for example, indication of traffic information such as a traffic sign and its meaning, other vehicles and their attitude, and the like altogether in addition to mere objective detection for avoiding collisions (that is, simple object detection), different from the aforesaid conventional vehicle with the millimeter wave radar traveling in expressways.
The millimeter wave radar, however, is basically a one-dimensional sensor (linear sensor) for detecting distance and relative velocity to an object. Therefore, no attempt to detect even such various indications is made.